The present invention relates to a method for measuring the oxygen content in a sealed target space, particularly for monitoring inertization levels in an inert gas device for fire prevention and/or fire extinguishing, and to a device for carrying out the method.
In closed rooms containing equipment that is sensitive to water, such as DP areas, electrical switching and distribution rooms, or storage areas containing high-value goods, what are known as inertization methods are increasingly being utilized to reduce the risk of fires and to extinguish them. The extinguishing effect produced by this technique is based on the principle of oxygen displacement. As is generally known, normal ambient air consists of 21% oxygen, 78% nitrogen, and 1% other gasses by volume. In order to extinguish and prevent fires, the inert gas concentration in the relevant space is elevated, and so the proportion of oxygen is reduced, by infusing an inert gas such as pure nitrogen that displaces oxygen. Many substances no longer burn when the oxygen level drops below 15–18% by volume. It may be necessary to lower the oxygen level further to 12%, for example, depending on the combustible materials in the relevant room.
Some known systems employ an inertization method with which a fire can be effectively extinguished given an optimally small storage capacity for the flasks of inert gas. According to this method, the oxygen content in the closed room is lowered to a base inertization level, (e.g., 16%) and in the event of fire, a very rapid full inertization occurs (e.g., an inertization of 12% or lower).
An inert gas device for fire prevention and/or fire extinguishing for carrying out the cited inertization method includes the following components: an oxygen meter for measuring the oxygen content in the monitored target space; a fire detector for detecting a combustion parameter in the air of the target space; a control for evaluating the data of the oxygen meter and the combustion parameter detector and controlling the running of the inertization process; and an apparatus for producing inert gas and abruptly infusing it into the target space.
The term combustion parameter refers to physical quantities that underlie measurable changes in the environment of an incipient fire (e.g., the ambient temperature, the proportion of solids, liquids, or gas in the ambient air (formation of smoke in the form of particulates, aerosols, or vapor) or the ambient radiation).
The oxygen meter serves for setting the base inertization level in the target space. If a threshold oxygen concentration value is exceeded, such as due to a leak in the target space, the control sends a command to a separate system to infuse inert gas into the space, so that the oxygen proportion is reduced. The oxygen meter signals when the threshold value of the base inertization level has been reached again. The position of the base inertization level therein depends on properties of the room. But if the detector for combustion parameters senses a combustion parameter, however, the system receives a command to flood the room with inert gas until the oxygen concentration in the target space is reduced to a specified full inertization level.
The measuring of the oxygen content in the target space is important for a reliable control of the method in this type of inert gas device for fire prevention and/or fire extinguishing. According to the prior art, the oxygen concentration in the target space is measured by point shaped oxygen sensors, which transmit the measurement values of the oxygen content to the control in the form of an analog signal. It is common to utilize 4–20 mA current interfaces, where 4 mA corresponds to a concentration of 0% oxygen, and 20 mA corresponds to the end of the measurement range (e.g., 25% oxygen). The disadvantage of utilizing point shaped oxygen sensors is that a greater number of such sensors are needed in the target space in order to get a representative reading of the oxygen content in the air in the room. That requires correspondingly costly cable connections between the individual sensors that are distributed in the target space and the actual control. Furthermore, the control requires a correspondingly high number of analogous interfaces. This requires a particularly large and particularly expensive hardware outlay.
An exceptionally disadvantageous aspect turns out to be that the control must continuously process a large number of signals. In particular, forming average values, estimating errors, and comparing to preset threshold values require routines, which are absolutely indispensable for controlling the inertization process. Only with the aid of the processed data of the oxygen sensors is it possible to drive the system for infusing inert gas, a fresh air supply, or a fan for air circulation in the target space. The signal processing in the control is therefore very intensive and requires a high complexity of software.